sudden drop of PH

[PEP12]

my PH which averages 8.25, with a daytime high of 8.33 dropping to 8.24 approx. preciseley at 09.30 each day it drops right down to approx. 8 for a minute or so then shoots back up to around 8.2, i have checked with charts on my Apex for any relation with temperature and ORP, nothing relates to the time drop, i have checked dosing time and the only one that coincides with it at 09.30 is the Red Sea NO Pox which starts at 09.00 and ends after 10ml at 10.00, any suggestions to what could be causing this?
one thing i should add is that there is no change to the tank area room lighting at that time,

 

[ZachR32]

It's the NO pox, I wouldn't worry. How close is the probe to the dosing outlet?

 

[PEP12]

the probe is 2 inches away

 

[ZachR32]

So the Nopox has weak acid in it, when it's added close to the probe it's directly affecting the pH close to the probe before it mixes fully into your tank. So the local pH appears much lower for a brief moment while the overall pH is still maintained